improve how markdown code language is commented

We added a space between the markdown triple code mark
and language that was enclosed in HTML comments.

So a line of the form:

````
```<!---c--->
````

became:

````
``` <!---c--->
````

This was done to **help** some code see that the HTML comment
string was not a markdown code specifier.   Not all code
understands this after this changed.  For example, `pandoc(1)`
still forms HTML classes of the form "&lt;!---c--&gt;"
when it encounters the above mentioned change.

We also fixed a problem with `2019/dogon/README.md` regarding
a markdown code block.

We also fixed a problem with `2020/yang/README.md` regarding
a markdown code block.

1990/dg/README.md

@@ -1,20 +1,20 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     echo foo bar | ./dg
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -24,13 +24,13 @@
 Some C preprocessors got confused about single quotes in certain cases.
 In particular, the following source line:
 
-```<!---c-->
+``` <!---c-->
     b12(ase(bco(acp(bi(acp(bcp(acp(bpl(aqu(bqu(A))))))))))))))))))))))))))))))
 ```
 
 sometimes has to be changed to:
 
-```<!---c-->
+``` <!---c-->
     b12(ase(bco(acp(bi(acp(bcp(acp(bpl('A'))))))))))))))))))))))))))))
 ```
 
@@ -56,7 +56,7 @@ As for the rot13 algorithm, it relies on `index(3)` to find if the
 character is in the set `A-Za-z`. `l[]` is an array that gets filled
 with:
 
-```<!---c-->
+``` <!---c-->
     "nopqrstuvwxyzzzzabcdefghijklmmmmNOPQRSTUVWXYZZZZABCDEFGHIJKLMMMM"
 ```